At present, composite films, which are produced by laminating films, such as a plastic film, a metal foil such as an aluminum foil, a metal deposited film, and a silica deposited film, by use of adhesive, are being widely used as flexible packaging material used for packaging materials in various industrial fields, including food products, beverages, medical products, and quasi-drugs, or electronic components including, for example, a hard disc.
Two-part reaction ester urethane adhesives which are used by combination of polyisocyanate and polyester polyol are being used most as the adhesive used for producing these composite films, because of their excellent adhesion properties.
In recent years, the studies on the material that may be eluted from the composite film into its contents have been advanced, indicating the possibility that a low-molecular-weight compound may be eluted from the adhesive into the contents to cause partial spoilage of inherent properties or performances of the contents, such as an odor and a taste of the food or the beverage, or the inherent performances of the electronic components, though the cause and effect relationship has not yet been clarified.
For example JP Laid-open (Unexamined) Patent Publications No. 2001-107016 and No. 2001-107017 proposed a laminate adhesive comprising polyester polyurethane polyol, wherein concentration of a cyclic ester compound or a cyclic urethane compound in extracted water, which is extracted from the composite film bonded by the laminate adhesive by using water of 0.5 mL/cm2 per unit area of the composite film is 0.5 ppb or less in terms of dibutyl phthalate concentration measured with a gas chromatograph-flame ionization detector so that the low-molecular-weight compound that may be eluted into the contents of the composite film can be reduced to prevent the inherent properties or performances of the contents from being spoiled.
The laminate adhesive proposed by JP Laid-open (Unexamined) Patent Publications No. 2001-107016 and No. 2001-107017 includes a dimer acid as composition of polyester polyol and, accordingly, it provides good water resistance, but disadvantageously decreases in oil resistance. It also decreases in concentration of an ester group and thus decreases in cohesion. Due to this, when this laminate adhesive is applied to the composite film for packing an oily food product, such as a retort food, requiring heat sterilization treatment, it disadvantageously reduces in strength of the composite film significantly after heat-sterilized.
It may be conceivable that for example an amide bond is introduced in polyester polyol to improve the cohesion. However, when the amide bond derived from aromatic or aliphatic bibasic acid used as a common ester material is introduced therein, flexibility of the adhesive required for the production of the flexible packaging composite film is spoiled and peel strength of the same is reduced disadvantageously.